The present invention relates to a pressure sensitive adhesive sheet to be used at the time of producing a thin type chip by processing a wafer on which a semiconductor circuit is formed. Further, the present invention relates to a method for producing the thin type chip by processing the wafer on which the semiconductor circuit is formed by using the pressure sensitive adhesive sheet.
In recent years, spread of IC cards has been promoted and further reduction of thickness thereof is now demanded. Accordingly, it is now required that the thickness of a semiconductor chip, which has been about 350 μm, is reduced to 50 to 100 μm or less. As a technology for attaining the reduction of chip thickness, a process for producing the semiconductor chip in which a groove of a given depth is formed from a surface side of the wafer and, thereafter, the wafer is ground from aback face side is disclosed in JP-A No. 5-335411. This process is known as a “predicing process” and can efficiently produce the thin type chip by simultaneously grinding the back face of the wafer (hereinafter, referred to also as “wafer back face grinding”) and dividing the wafer into individual chips.
At the time of performing the wafer back face grinding by the predicing process, in order to protect the circuit on the surface of the wafer or to fix the wafer (chip), a specific pressure sensitive adhesive sheet is stuck onto the surface of the wafer on which the groove is formed. The present inventor has solved various types of problems generated in the course of producing the thin type chip by the predicing process by developing a unique pressure sensitive adhesive sheet.
For example, in Japanese Patent No. 3410371, the number of chip cracks to be generated at the time of performing the back face grinding is reduced by specifying compressive strain of the pressure sensitive adhesive sheet and elastic modulus of a pressure sensitive adhesive layer. In JP-A No. 2001-127029, an internal stress generated at the time of sticking the pressure sensitive adhesive sheet to the wafer is relaxed and narrowing of a chip-chip space which becomes a problem at the time of dividing a wafer into individual chips in a grinding step is prevented by using the pressure sensitive adhesive sheet which shows specified Young's modulus and stress relaxation ratio. Further, in JP-A No. 2003-147300, production of an extremely thin chip by the predicing process is allowed to be adapted to an automatic process by enhancing an adhesion property to a stripping tape by means of providing a constituent layer having a low glass transition point in a substrate of the pressure sensitive adhesive sheet.
Reduction of thickness of the wafer (chip) has been progressed in the predicing process by these inventions. As a result, a new problem has appeared. As the wafer was ground thinner, the wafer came to be more brittle. Then, a breakage was generated in a portion at any one of four corners of a produced chip due to vibration or impact transmitted from a grinding stone during a grinding operation. Further, since stress at the time of further grinding a thin wafer was not evenly applied to the thin wafer, discoloration has appeared on a thus-ground face of the wafer.